Thorn/BRC 2000

[ppppenguin]

Many thanks. I suspect the problem with BU208 will be low beta. I found this when fixing a Prowest monitor a few years ago. This had a duff LOP transistor which was an odd type like BU102 or BU104. These didn't have a very high voltage rating but did have a beta of about 30 rather than the 10 or so for a BU208.

First step is to try the other board. Change its Callins caps and replace a couple of missing parts.

[Murphyv310]

Hi Jeffrey.
I've NOS BU102 5 of them and one BU100A. There's a couple of used BU205 as well.

[ppppenguin]

I'll try the other board first. BU102 might be OK, VCBO is 400V. BU205 has ample ratings but beta might be too low. Never heard of BU100A, must be very early. From memory the classic LOP transistor was the BU105, used in pairs in many of the early sets.

[Murphyv310]

You are welcome to the BU102's FOC if you want them. I can post them tomorrow if you wish.

[ppppenguin]

Thanks again but let's hang on and see if my other board comes good.

[Refugee]

I might have some of these insulating kit free devices.
There is also a DF type with the fly-back diode fitted internally.
http://pdf1.alldatasheet.com/datasheet-p...525AF.html

[Murphyv310]

Hi.
Loads of more "recent" line OP Transistors here. The AF versions don't have the Diode, Ref, you need the *D* versions for the diode.
I'd be reluctant though to use transistors with the integral diode in a 2000.
The BU100 was as far as I know is the first generation Philips Silicon line output transistor for CTV, that begs the question for what application?? The BU102 was its successor. The Philips G8 which was released in 1970 used the BU105 initially.

One last thing, I don't really think beta is a big issue in the 2000, the transistor is acting as a switch and the drive should still be able to turn on a BU208 hard enough, if the beta is way too low it will overheat and fail within a few minutes, there could be lack of width or foldover left of centre on the picture before the transistor fails.

I'm 90% certain I've fitted BU205's in a 2000 and possibly 208's.......... it though isn't yesterday and I may be wrong.

[Refugee]

I did say "a DF type with a diode".
They may be a tad fast for the 2000 but adding a little padding to the fly-back capacitor would seen fix it if there is any line stretching.
Did those early thorn sets not have some kind of push pull circuit in them?

[ppppenguin]

One last thing, I don't really think beta is a big issue in the 2000, the transistor is acting as a switch and the drive should still be able to turn on a BU208 hard enough, if the beta is way too low it will overheat and fail within a few minutes, there could be lack of width or foldover left of centre on the picture before the transistor fails.

I didn't think beta was an issue in LOP transistors until I got caught out. In the Prowest monitor I was fixing there just wasn't enough base drive to saturate a BU208 at the right hand edge of the scan. It was in a separate EHT generator, much like the 2000, so there were no visible effects on the screen.. The symptoms were excess current consumption and the transistor getting hot. A careful look at the collector waveform showed what was happening. Normally the scope gain is turned well down to see the flyback pulse (about 200V on that design) but if you looked carefully at the baseline and turned up the gain you could see the collector voltage rising as it came out of saturation.

No way of knowing if the 2000 has enough base drive for a BU208 without trying it.

[Murphyv310]

I might have some of these insulating kit free devices.
There is also a DF type with the fly-back diode fitted internally.
http://pdf1.alldatasheet.com/datasheet-p...525AF.html

So you did. Just the link that was confusing as it's for the AF types.